The preparation of sulfur dioxide by the reaction of sulfur and sulfur trioxide in an oleum medium is shown in Swiss Pat. No. 477,364. The reaction is carried out at a temperature of 40.degree.-50.degree. C. and is said to produce sulfur dioxide having a purity of over 99.5% and that this purity can be raised to 99.95% by passing the sulfur dioxide through a secondary reactor filled with sulfur particles. Sulfur dioxide of this purity does not meet most commercial specifications and must be further refined for most purposes.
It has now been found, according to the invention, that by raising the temperature at which the reaction is run to 100.degree.-150.degree. C. the rate of the reaction is greatly increased, and that this, together with rectification of the reactor effluent, raises the purity of the sulfur dioxide product to a level which permits it to be used directly in most commercial applications without further purification.
To have these advantages result from raising the reaction temperature is completely surprising because the Swiss patent tells us that at temperatures of 40.degree.-50.degree. C., the reaction proceeds just as quickly as it does at, for example, 90.degree. C.